1. Field of the Invention
The present invention relates to local, wide area, and national telecommunications networks and particularly relates to telecommunications networks using fiber optic technology.
2. Description of the Related Art
Over the past two decades, the rapid deployment of local and long-haul transmission systems using single mode optical fibers has had profound impact on the quality, utility, and variety of telecommunications equipment and services. The emergence of the optical fiber as the transmission medium of choice stems from several key attributes: low loss, low dispersion, and low cost (silica, the essential raw material, is abundantly available). Collectively, these fiber optic attributes permit point-to-point movement of greater volumes of information over greater unrepeated distances, when compared against other types of transmission media such as copper cabling or radio. Local and Metropolitan Area Data Networks such as the Fiber Distribution Data Interface (FDDI), efficient subscriber loop systems, and the Synchronous Optical Network specification for core network equipment standardized from the physical through the Operations, Administration, and Maintenance (OA&M) layers are but a few examples of the range of capabilities enabled by optical fiber-based transmission systems. The availability of the Erbium Doped Fiber Amplifier (EDFA), described in T. E. Bell, "Technology 1991: Telecommunications," IEEE Spectrum, Vol. 28, January 1991, has only served to heighten the appeal of these systems.
More recently, considerable interest has developed with regard to another, as yet untapped, attribute of single mode optical fiber: its enormous bandwidth. Even if we conservatively confine our attention to the narrow optical window centered around 1.55 microns coinciding with the operational range of the EDFA, a potentially addressable bandwidth of about 5 terahertz exists. This enormous potential has suggested the use of multiwavelength technology to increase the capacity of each fiber by a factor of at least 10, and perhaps by as much as 100.
If each wavelength-multiplexed channel is operated at the order of a multigigabit/sec. rate, then a single fiber link may provide a capacity which is much greater than that needed on a point-to-point basis. The objective, then, is not merely to magnify the capacity between a single transmitter/receiver pair but, rather, to create an optical medium containing many interconnected fibers such that each wavelength injected by a single transmitting station is independently routed to one of many distinct receivers. The medium would also contain optical power splitters, combiners, wavelength selective filters, and wavelength multiplexers and demultiplexers, and would provide the basis for a multiuser lightwave network, as opposed to an optical transmission system. When coupled with techniques for re-using the same wavelengths on disjoint fibers and with the deployment of EDFAs to offset optical combining and splitting losses, an approach such as this may provide a limitless supply of information-bearing wavelength multiplexed channels. The lightwave network, then, becomes a viable platform for a new wide-area telecommunications infrastructure, scalable to very large configurations both in geographical size and number of users served, and capable of providing a plethora of bandwidth-intensive multimedia services to large and small users alike.
In the telecommunications context, a lightwave network might appear as shown in FIG. 1, and include a shared optical medium 100, and geographically dispersed access stations (1 through N, where N is an integer) which contain the ports 101 through which users attach to the network. Here, each user might be a processor, advanced workstation, video monitor, local data network, central office, or any other type of telecommunications apparatus. The signals injected by each user, which might already be highly aggregated, must each be delivered by the lightwave network to its intended destination. Each access station is responsible for converting the applied user signals into an optical format, placing these signals onto the medium, retrieving from the medium those signals intended for itself, and delivering these to the attached user.
The optical medium of FIG. 1 is capable of performing only linear optical operations such as signal combining and splitting, wavelength selective filtering, and linear amplification, and is totally devoid of logical functionality. Its sole responsibility is to deliver each wavelength-multiplexed signal (.lambda..sub.1 through .lambda..sub.10) injected by each access station to its intended receiver. All logical operations needed to create a telecommunications signal transport capability, such as routing and protocol processing, are performed by electronic circuitry in the access stations. The access stations, then, are also responsible for electronic processing of user signals prior to their placement onto the optical medium and subsequent to their retrieval from the optical medium. The medium, therefore, provides the optical channels which interconnect the access stations, and the access stations prepare user signals for transport over the medium.
In a certain functional sense, the lightwave network of FIG. 1 is no different from a conventional local area network (LAN). The LAN consists of a shared medium, along with access stations which transfer user signals among themselves over the shared medium. There are, however, two important distinctions. First, whereas the LAN medium typically involves a single high-speed channel which is shared by all access stations, the medium of the lightwave network involves a great number of wavelength multiplexed/wavelength re-used high-speed channels. Second, whereas the LAN serves a small population of users scattered throughout a small service domain (typically less than 1 km. in diameter), the lightwave network is potentially capable of creating the wide-area infrastructure needed to provide primary telecommunications services, to a vast user population.
The interest in lightwave networks arises from several observations and prospective opportunities. First is the enormous information-bearing capability of such a network. This capacity can be used to deliver advanced services to users or to enable execution of bandwidth-intensive applications. Part of its capacity might be allocated to the execution of a rich set of self-diagnostic routines, thereby improving network reliability as observed by the user. Yet another part of this capacity might be "squandered," in the sense of simplifying network management and control software at the expense of consuming greater capacity than would otherwise be needed to meet aggregate user demand. Second, such networks are both modular and "hardware scalable"; the network contains only the medium and the access stations, and larger networks can evolve from smaller networks by deploying more of the same types of equipment. Third, the network supports clear channel connections among the access stations, each of which can be used to carry information in an arbitrary transmission format. Moreover, it is possible to overlay a fast, packet switching capability on top of these clear channels, thereby enabling complete connectivity among the access stations at the virtual connection level, despite their limited connectivity at the optical channel level. Fourth, such a network is readily reconfigurable. Referring back to FIG. 1, we note that by re-assigning wavelengths differently among the access stations (1 through N), it becomes possible to alter the clear-channel optical connections among them, a property which is important when re-deploying resources in response to changing traffic patterns or reconfiguring in response to network equipment failure and recovery.
Interest in lightwave networks first developed in the mid-1980s. At that time, optical transmission systems were already firmly entrenched, and leading researchers began to seek new directions for applying optical device technologies. The enormous bandwidth prospects of single mode optical fiber soon presented an inviting target, and much device-level research was pursued, focusing on the production of multiwavelength technologies needed to create a suitable physical layer. Produced were wavelength multiplexing techniques, advances in semiconductor lasers, tunable optical filters, coherent optics technology, and more sensitive optical receivers. Also produced were techniques and subsystems for stabilizing the closely-spaced wavelengths generated by a comb of single frequency laser diodes. Work was also pursued in the area of electronically controlled optical waveguide switches based on Lithium Niobate technology. Work focused on the creation of rapidly tunable laser diodes was initiated, an effort that persists today. Efforts were also begun to develop optical amplifiers and electronically controlled wavelength-selective optical switches. These latter efforts have brought forth two technologies important to the subsequent development of lightwave networks: the Erbium Doped Fiber Amplifier and the Acousto-Optical Tunable Filter (AOTF).
Systems efforts during this early era were focused on a simple architecture: the broadcast and select network. A block diagram of such a network appears in FIG. 2. Here, each of access stations 1 through N is equipped with a single laser diode transmitter 200 capable of generating light at a unique wavelength, and with a single optical receiver 201 capable of being tuned to the wavelength of any one such transmitter. Signals produced by the transmitters are linearly superimposed in a centrally located optical star coupler 202, a passive device which broadcasts a scaled version of the superimposed signals back to each receiver. By selecting the appropriate wavelength, each receiver can accept the signal injected by the corresponding transmitter, thereby creating a clear channel from the transmitter to that receiver. In an alternative arrangement, each receiver accepts light at a fixed, predefined wavelength, and each transmitter is made wavelength-agile and capable of creating a channel to any receiver by tuning its wavelength to that accepted by the chosen receiver.
To be of use as a broadband multimedia network, it is necessary that the user connected to each access station of a broadcast and select network be capable of maintaining an independent dialogue with each of several other users. This is especially true if the signals injected by a generic optical network user are already the aggregate of signals generated by other, lower-speed users (e.g., one type of generic optical network user may be the gateway node of a LAN).
In a typical asynchronous transfer mode (ATM) network, the capability of providing independent communications is handled by converting each signal produced by a given user into a sequence of fixed-length ATM cells, with the destination for each cell being identified by the virtual connection number contained in that cell's header field. The cells produced by a given user corresponding to different virtual connections 1 through M (and, most likely, intended for different recipients) are statistically time-multiplexed by statistical multiplexer 300 prior to presentation to the network, as shown in FIG. 3. The network then reads each header of the resulting sequence of cells, and delivers each cell to its intended receiver. All forms of telecommunication traffic (voice, data, image, video) must first be converted to sequences of ATM cells. In such a fashion, each user can simultaneously maintain an independent dialogue with each of some multitude of other users.
To provide such a capability, each receiver (or transmitter) of the broadcast and select network must be capable of retuning to a new wavelength, once per each ATM slot time. A simple three input/three output example appears in FIG. 4, including transmitters 401, 402, and 403, and receivers 410, 411 and 412, with wavelength agility provided at the receivers 410, 411 and 412. Referring to the output at receiver 410, for example, we note that, in rapid sequence, it must retune from .lambda..sub.3 to .lambda..sub.2 and back to .lambda..sub.3. Such rapid "packet switching" presents several major barriers. First is the difficulty of tuning, itself. Each receiver (or transmitter, if that is where the wavelength agility resides) of an N-station network must re-select the appropriate 1-out-of-N wavelengths very rapidly in the narrow guard time between ATM cells. By way of example, if the data rate of each optical channel is 1 Gigabit/sec., then the ATM cell, which contains 53 bytes, has a duration of 424 nsec.; allowing for some silent guard time between consecutive cells, a slot size of perhaps 500 nsec. results. Re-tuning must therefore be done once every 500 nsec., with the re-tuning transient completed within about 75 nsec., maximum, and the newly chosen wavelength must then remain stable for the subsequent 424 nsec. While this may be possible for small values of N (say, under 10), there remain the issues of both short- and long-term drift of the chosen wavelength. Second, referring to FIG. 4, it is necessary that the cells generated from the various access stations have time-aligned time slot boundaries, as they arrive at the star coupler 420, which further suggests that the star coupler be implemented as a single, centrally located element. If this is not done, then the receivers will necessarily miss portions of the arriving cells since only a single wavelength can be chosen at a time. Third, again referring to FIG. 4, it is apparent that careful coordination is required among the transmitters 401, 402 and 403, such that, for each time slot, each transmitter chooses a distinct receiver (if two transmitters were to choose the same receiver at the same time, then one of the transmitted cells would be lost since, again, a given receiver can choose only one wavelength at a time). Furthermore, by some mechanism, each receiver must be told, slot-by-slot, which wavelength to choose. Although many strategies have been proposed for such signaling means, few are useful if the network is large because of the large volume of signaling information involved (e.g., a hypothetical 8,192-station network would require a signaling rate of 8,192 stations at 13 address bits per station every 500 nsec.=213 Gigabits/sec.!). Although scheduled time division multiplexing can avoid the need for such signaling, this produces a rigidly defined bandwidth for each user-to-user connection, which may be inappropriate for multimedia networks integrating continuous bit rate and bursty variable bit rate traffic. Scheduling and transmission coordination can also be avoided if wavelength agility is implemented at the transmitter, but these are replaced by the likelihood that collisions will occur if two or more transmitters attempt simultaneously to transmit on the same wavelength to the same receiver (this is an ALOHA-like random access solution, and would also involve collision detection mechanisms, backoff/re-transmission strategies and, at best, 36% channel utilization efficiency). Finally, the number of usable wavelengths is limited by the sophistication of the wavelength multiplexing technology employed and the 5 terahertz passband of the EDFA which is required on each output port of star coupler 420 to offset optical power splitting and insertion losses. If adjacent optical carriers are separated by 1 nm. in wavelength, then the number of wavelengths and, hence, the number of access stations would be limited to approximately 100. Thus, we conclude from the above that the simple broadcast and select approach, which was the focus of lightwave network research at the dawn of the field, does not provide a suitable platform for a new, scalable, wide-area telecommunications infrastructure.
As the potential of lightwave networks became more apparent, telecommunication systems researchers became attracted to the field, joining the device researchers who had originally identified the opportunities and who had made solid advances at creating an arsenal of device and subsystems capabilities. A new concept, known as multihop, was soon suggested, which provided the basis for much ongoing systems-level research on lightwave networks. The multihop approach avoids many of the problems associated with the aforementioned broadcast and select approach. When combined with new devices capable of supporting wavelength re-use technologies, the systems-level concepts associated with multihop are readily extended to create a scalable hardware platform which may serve as the basis for a new wide-area infrastructure.
The multihop approach, illustrated in FIG. 5a for an eight-station system and a physical bus topology, provides a fast packet-switching service (which may, in fact, conform to ATM) while avoiding the need for rapid, cell-by-cell wavelength agility. For the example illustrated in FIG. 5a, each access station 501 through 508 is equipped with two fixed-wavelength transmitters and two fixed-wavelength receivers (not shown in FIG. 5a); no two transmitters share the same wavelength, and no two receivers share the same wavelength. Signals are injected onto the transmit bus 510 via directional couplers (not shown in FIG. 5a), and the superimposed signals, each distinguished by its unique wavelength, are broadcasted onto the receive bus 520.
By means of its two optical transmitters, each access station creates a different clear optical channel to each of only two other access stations. Thus, the access stations are not fully interconnected at the optical channel level, and a wavelength incompatibility problem arises if a given access station intends to send a cell to one of the stations to which it does not have a clear channel (an overwhelmingly likely situation if there are many access stations, each generating and terminating only two clear channels). Resolution of this wavelength incompatibility problem requires wavelength translation, a function easily accomplished in the electronic domain if we require that each access station, in addition to connecting a user to the network, also serves as a cooperating relay station for packets introduced by other stations. For example, if station 501 in FIG. 5a wishes to send an ATM cell to station 502, it might present this cell to the medium using .lambda..sub.1 ; after propagating once past head-end 530, this cell will arrive at station 505, having the only receiver listening to .lambda..sub.1. Using its electronic circuitry, station 505 recognizes that this cell is intended for station 502, and (again in electronics) routes this cell to its transmitter which operates at .lambda..sub.10. After a second pass over the medium on .lambda..sub.10 (hence, the name "multihop"), the cell arrives at station 502. Note that the wavelength incompatibility problem was solved without requiring direct optical-to-optical wavelength translation but, rather, involved an interim conversion of the ATM cell to electronics within access station 505. The cell was then processed by a small 3.times.3 electronic ATM switch (one user input, two inputs from the optical medium; one user output, two outputs back onto the optical medium). Contention which may arise if multiple cells should concurrently arrive at an access station, all bound for a common output from that station, is resolved by means of store-and-forward smoothing buffers residing in the ATM switch where contending cells are temporarily stored and delivered sequentially to the output.
For the example presented in FIG. 5a, it can be shown that the assignment of wavelengths among the access stations allows any station to reach any other station in, at most, three hops. The actual wavelength assignment, showing the originating and terminating station of each wavelength, appears in FIG. 5b in the form of a so-called optical connectivity diagram. For the example given, this connectivity diagram takes the form of a recirculating perfect shuffle in which the stations are arranged in two columns, each containing four stations. The transmitters (not shown in FIG. 5b) of column 1 stations 551, 552, 553 and 554 are connected to the receivers (not shown in FIG. 5b) of column 2 stations 561, 562, 563 and 564, and the transmitters (not shown in FIG. 5a) of column 2 stations 561, 552, 553 and 554 are connected to the receivers (not shown in FIG. 5a) of column 1 stations 551, 552, 553 and 554. Using the optical connectivity diagram, it is possible for the network admission controller (not shown) to choose a path (a sequence of optical links) for a newly requested virtual connection originating at any given station and terminating at any other. As with a conventional ATM network, a multiplicity of virtual connections may be multiplexed onto each link and, at call admission time, the admission controller determines whether the inclusion of the traffic presented by the newly requested connection will unacceptably degrade the quality-of-service enjoyed by other virtual connections sharing any common link with the newly requested connection. If so, the new connection request is denied; if not, the new connection is admitted and the routing tables in the ATM switches encountered along the chosen path are provided with the appropriate instructions. The difference here is that, for the lightwave network, each link corresponds to a different wavelength, with all such wavelengths multiplexed onto a common physical medium, whereas for a conventional network, each link corresponds to a unique point-to-point transmission system with its own dedicated optical fiber medium.
While not recommended for a scalable wide-area telecommunication infrastructure, the recirculating perfect shuffle connectivity diagram of FIG. 5b is well-suited to uniform traffic patterns--i.e., those having a common traffic intensity between each pair of users. One of the problems associated with the recirculating perfect shuffle diagram is that it is not modular but, rather, the number of access stations must obey the relationship EQU N=kp.sup.k, (1)
where p is the number of optical transceivers per access station (p=2 for FIG. 5) and k is the number of columns in the connectivity diagram, each containing p.sup.k stations. A shufflenet is a multihop network using a recirculating perfect shuffle connectivity diagram having a number of access stations characterized by Formula (1). An N station shufflenet requires a total of Np optical links. For such a network, it can be shown that under conditions of uniform traffic loading that the aggregate network capacity, defined as the maximum exogenous load which can be accepted such that the total load (new traffic plus relay traffic) on each link is equal to the link capacity, is given by: ##EQU1## where S is the capacity of each optical link. Formula (2) is gotten by dividing the sum total capacity of all optical links (N.times.p.times.S) by the expected number of hops taken by a representative packet. The expected number of hops is approximately log.sub.p N when N is large. By dividing C.sub.T by N, we can compute the maximum traffic load which can be offered by each access station.
Note that the connectivity diagram of FIG. 5b bears no resemblance to the physical topology of FIG. 5a. Note also that, since each optical channel is a separate entity having a single transmitter and a single receiver, there is no need to maintain time slot synchronization as was the case with broadcast and select networks.
Corollaries of these observations include first, that any physical topology which broadcasts the combined optical signals can serve as the basis for a multihop network. For example, the star topology of FIG. 4 can be used. Second, any optical connectivity diagram can be produced, subject only to the constraints that each station can generate and terminate only p optical channels. Third, a multihop network need not be based on a recirculating perfect shuffle connectivity diagram. Any reasonable connectivity diagram will do. In fact, under conditions of non-uniform traffic, a connectivity diagram which creates clear channels between the stations-pairs presenting the greatest traffic intensities will always outperform a recirculating perfect shuffle diagram. Fourth, the number of stations in a multihop network is not bound by Formula (1), since any reasonable connectivity diagram will suffice; Formula (2) presents a constraint only on shufflenets, but not on multihop. Fifth, if the transmitters or receivers of the multihop network access stations can be tuned to different wavelengths, then it becomes possible to alter the connectivity diagram, while leaving the physical topology unchanged. For example, see J. F. Labourdette and A. S. Acampora, "Logically Rearrangeable Multihop Lightwave Networks," IEEE Trans. Comm. Vol. COM-39, August 1991. Note that rapid agility is not needed here. This allows the connectivity diagram to be updated whenever traffic patterns change and whenever a new access station is added to the network. A multihop network with tunable transmitters or receivers is therefore modular. Furthermore, just as the connectivity diagram can be changed to admit a new access station, it can also be changed to bypass a failed access station. Sixth, unlike the broadcast and select network which required rapid wavelength agility, the reconfigurability property of the multihop network can be capitalized with greatly relaxed requirements on tuning speed. Here, tuning is not involved on a packet-by-packet basis but, rather, only when a change of connectivity diagram is desired. In general, slow tuning over a broad optical window is much more feasible. Seventh, in essence, slow tuning provides a circuit switching feature whereby an appropriate connectivity diagram is produced. Then, multihop provides the required packet-by-packet wavelength translation needed to provide full connectivity at the virtual circuit level, despite decidedly limited station-to-station connectivity at the optical channel level.
One major problem yet remains: the number of wavelengths which can be multiplexed onto a common physical medium is limited, as before, to the number which can be placed in the passband of the EDFA. However, wavelength re-use can be invoked to overcome this limitation, and electronically controlled wavelength-selective routing can eliminate the need for any transceiver wavelength agility, while still preserving the reconfigurability properties of multihop networks.
The production of a multihop connectivity diagram for a large network requires a large number of optical clear channels (p.times.N for an N-station network, each with a fanout of p). These channels need not be wavelength multiplexed onto a common medium; it is possible to create a multihop network by means of re-using a common set of wavelengths on multiple media.
A possible approach is shown in FIG. 6. Here, wavelengths .lambda..sub.1 -.lambda..sub.8, used by access stations 601, 602, 605, and 606 on Transmit Bus 610, are also used by stations 603, 604, 607, and 608 on Transmit Bus 620. By means of passive wavelength demultiplexers and re-multiplexers 651, 652, 653 and 654, wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.5, and .lambda..sub.6 from Transmit Bus 610 are combined with wavelengths .lambda..sub.3, .lambda..sub.4, .lambda..sub.7, and .lambda..sub.8 from Transmit Bus 620 and are injected onto Receive Bus 630. Similarly, wavelengths .lambda..sub.3, .lambda..sub.4, .lambda..sub.7, and .lambda..sub.8 from Transmit Bus 610 are combined with wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.5, and .lambda..sub.6 from Transmit Bus 620 and are injected onto Receive Bus 640. The connectivity diagram of FIG. 7 results. Note that, although the physical topologies of the networks of FIGS. 5a and 6 are different, functionally identical optical connection diagrams have been produced. An advantage of the network of FIG. 6 is that only eight wavelengths are used. However, a disadvantage of the network of FIG. 6 is that the reconfigurability of the optical connection diagram is limited because the path of each wavelength between the demultiplexers and multiplexers is "hard wired"; for example, it is not possible to inject wavelength .lambda..sub.1 from Transmit Bus A onto Receive Bus B. For small networks, this lack of reconfigurability can be partially offset by exploiting slow tuning; since it is not possible for wavelengths .lambda..sub.1 and .lambda..sub.2 generated by Station A to be received at any of stations 3, 4, 7, or 8, it may be possible to retune Stations A's transmitters to allow such a connection, if needed (any one among wavelengths .lambda..sub.3, .lambda..sub.4 .lambda..sub.7, or .lambda..sub.8 would suffice). The other transmitters attached to Transmit Bus A would then be appropriately re-tuned.
A better way to maintain a high degree of reconfigurability while allowing a limited number of wavelengths to be re-used on different media is by means of wavelength-selective, electronically controlled optical switches such as those described in D.A. Smith, et al. "Integrated Optic Acoustically Tunable Filters WDM Networks," IEEE J. Sel. Areas Comm., Vol. 8, August 1990. A functional representation for such a 2.times.2 switch appears in FIG. 8. Here, two wavelength-multiplexed signals arrive on each of the two input fibers 801 and 802 and, under electronic control, the switch 800 has been instructed to combine .lambda..sub.1.sup.A, arriving on input 810 with .lambda..sub.2.sup.B, arriving input 820, and to deliver the combined signal to Output 830. Similarly, .lambda..sub.2.sup.A has been combined with .lambda..sub.1.sup.B and delivered to Output 840. For each of the wavelengths appearing on its two inputs, the switch 800 can be independently configured in the "bar" state (upper input connected to upper output, lower input to lower output), or in the "cross" state (upper input to lower output, lower input to upper output). Since the switch changes state under electronic control, only linear operations are performed on the optical signals (wavelength filtering and routing). Such a switch is therefore an admissible element of the "passive" optical medium.
Switches such as those just described can be fabricated by means of Acousto-Optic-Tunable-Filter (AOTF) technology, and 2.times.2 switches can be cascaded to create M.times.M switches (state-of-the-art technology currently limits AOTF switching matrices to, approximately, 8.times.8, and can independently route, approximately, 8 wavelengths). Note that the switching transient for such switches can be made small (on the order of microseconds).
The use of AOTF switches allows a very limited number of distinct wavelengths to be reused an arbitrarily large number of times, while maintaining a high degree of reconfigurability among the access stations. A high-level block diagram of a new, lightwave network-based, wide-area telecommunications infrastructure might then appear as shown in FIG. 9. A total of p wavelengths are used, and each access station 1 through N is equipped with p optical transceivers (not shown), one for each wavelength. The access stations connect to the optical medium 901 through M.times.M AOTF switches 910. All inputs/outputs of all switches contain all p wavelengths, and the switches are interconnected among themselves by means of single mode optical fiber, thereby creating a reconfigurable, wavelength-routed optical medium.
The network architecture presently being considered by those in the telecommunications field for a new, scalable, broadband telecommunications infrastructure is based on (1) the use of wavelength division multiplexing (WDM), (2) the use of wavelength to route each signal to its intended destination in the network (Wavelength Routing), (3) the translation of signals from one wavelength to another at the access stations (Wavelength Translation), and (4) the use of multihop packet switching. These four principles permit networks to be built whose size is essentially unlimited and independent of the number of wavelengths available.
The network consists of (1) an all-optical inner portion which contains the wavelength routing crossconnection elements, (2) user access stations which attach to the optical medium, and (3) a distributed network controller resident in the access stations.
It is instructive to represent the proposed infrastructure by the layered diagram of FIG. 10, with each layer providing a "service" to the one above it such that desired user-to-user connections can be made "on demand." At the lowest layer 1000 is the physical deployment of the medium 1001: the actual layout of the fibers, the routes over which population centers are interconnected by fiber bundles, the location of the AOTF switches, the access stations, and the connection of generic "users" to their access stations. We will not be further concerned with this level, except to say that topologies for the higher functional layers of the network are independent of the layout of the "trenches" in which the physical apparatus is installed.
The next layer corresponds to the fiber interconnection graph 1010. This layer defines how the optical fibers 1011 should interconnect the optical switches (not shown). Any desired fiber interconnection graph can be realized within the trench topology by conceptually stretching and distorting the fibers and the graph so that they fit within the trenches. At this layer, we see the users, their access stations, and the wavelength-selective optical switches, but not the trenches themselves. The graph can be matched to geographical traffic density and should not unnecessarily block desired access station-to-access station optical connections.
The next layer shows the optical connectivity graph 1020, corresponding to the optical channel connections 1021 which exist among the user access stations 1 through N (e.g., an optical channel using wavelength .lambda..sub.1 originating at station i, is routed through a series of AOTFs and terminates at station j, corresponding to a directed optical channel connection from i to j). At this layer we see the users and their access stations 1 through N, but not the trenches and optical switches; the optical connection graph shows how the access stations are connected by "clear channels."
Whereas the first two layers are not reconfigurable, the wavelength-selective optical switches provide the means to change the access station connectivity diagram. The connection pattern at this plane can be rearranged by altering the states of the AOTFs in response to significant traffic demand changes. Each optical connection is realized by assigning transmitters and receivers from the access stations and communication capacity from the fiber connection layer. Connectivity at this level, however, is limited, in that no more than p directed optical connections can simultaneously originate from or terminate at any access station. The optical connectivity layer is designed to satisfy a set of user traffic requirements (in the form of estimated throughput and desired quality-of-service) and to respond to traffic pattern changes or equipment failure/recovery.
As indicated above, the optical connectivity among the access stations is incomplete, due to the limited number of transmitters and receivers per access station. Most user-to-user connections must go through several optical channels to reach their destinations. The virtual layer 1030 represents the arrangement of virtual connections 1031 among the actual users and, here, each user could, in principle, have a virtual connection to every other user attached to the network. At this layer, we see only the users; the access stations, optical switches, and trenches are invisible and provide only the underlying support for user-to-user connections. Information carried by a particular virtual connection travels along a sequence of optical channels, with full connectivity provided by using intermediate access stations as cooperating relay stations. Virtual connections are created and torn down by the network in response to instantaneous user-to-user calling patterns. For example, if the lightwave network is used to support ATM, then the access stations would contain ATM switches. Virtual paths between pairs of end points would be set up as end-to-end paths through a series of ATM switches, traversing several optical channels--that is, they would be set up as "multihop" connections. The job of the admission controller is to find, in response to a call request, a virtual path (set of optical links from sender to receiver) capable of handling that call. If such a path cannot be found, the call would be blocked. At each step along the way, the call processor must check whether or not the traffic intensity corresponding to the new call request can be accommodated without unacceptably degrading the Quality of Service enjoyed by other existing connections.
Assuming that each generic user attaches through its own dedicated access station, and that each access station attaches to the medium through its own dedicated wavelength-selective switch, an "access module" 1100 may appear as shown in FIG. 11. A total of p wavelengths are re-used throughout the network, and each access station 1101 is equipped with p transceivers, one for each wavelength. Each access station also contains a processor which is used for network management and control. Each wavelength-selective switch 1100 contains M+1 input/output fiber ports (one port connects the access station; each of the remaining M ports connects to another switch). All P wavelengths exist on all fibers. In total, the network interconnects N users through N access stations and requires N switches. To add a new user 1102, one additional switch 1103 is needed, along with one additional access station 1101. Physically, the switch is added by (wisely) choosing M fibers from the existing plant, severing each fiber so chosen, and connecting the several ends of each fiber to one I/O port of the switch. Some of the optical channels generated by a given access station may be used in the "clear channel" mode and are routed through the switches to their intended receiving access stations. Others are used to create the multihop network and are routed to their intended access station receivers as defined by the multihop optical connectivity graph.
This model is scalable and modular in the hardware sense: large networks evolve from smaller ones by introducing additional copies of the generic access module; the number of modules is directly proportional to the user population, and the network can grow in size without bound (in both number of users and geographical span of coverage). However, we see that the underlying fiber connection graph is not rearrangeably non-blocking, in that an arbitrarily chosen optical connectivity graph may not be realizable. To insure that any arbitrary optical connectivity graph can be supported, the fiber graph would require, at least, on the order of N log N switches; since the actual number of switches is limited to N (by the scalability requirement), not all possible optical connectivity graphs can be supported. However, since each access station enjoys a full complement of P transceivers, this restriction may not be encountered until N grows quite large. In any event, this restriction may eventually cause some clear-channel blocking or preclude the realization of some desired multihop connection graph.
Therefore, it would be desirable to have a telecommunications network which is scalable and modular in both the hardware and software sense.